1. Field of the Invention
The present invention relates to a structure joining intersecting fiber based composite structural members of an airframe and the like, and to a process for making said structure.
2. Description of the Prior Art
Fiber based composite materials have been known in the prior art for a long time. Briefly, such materials comprise a plurality of relatively thin fibers and a reinforcing cured plastic which substantially covers the fibers and holds them together. Furthermore, it was recognized in the prior art that the structural strength of fiber based composite materials is the greatest in the direction of the fibers. Accordingly, composite materials have been prepared in the past wherein all of the fibers are disposed in one direction parallel to one another. These types of composite materials are hereinafter referred to as unidirectional composite materials.
A principal characteristic of unidirectional fiber based composite materials is their above-mentioned anisotropy. Thus, these materials exhibit relatively great strength to withstand forces which are applied substantially in the direction of the fibers. However, the load bearing or force withstanding capability of the unidirectional fiber based composite materials against forces which are applied perpendicularly to the direction of fibers, is substantially less. This follows from the fact that the parallel disposed fibers of the composite materials are held together only by the cured resin. The prior art composite materials or structures are particularly vulnerable to forces which tend to separate the fibers from one another in a direction perpendicular to the layout of the fibers. Nevertheless, composite materials, and particularly plastic reinforced glass fibers (fiberglass) have found several applications in the prior art where a relatively light weight and yet strong structural material was desired.
The relatively recent development of unidirectional composite materials containing graphite and other fibers of high strength has rendered possible the utilization of fiber based composite materials in airframe construction. More particularly, unidirectional composite materials comprising epoxy resin reinforced graphite or other fibers of high strength are currently used, at least to a limited extent, to provide stringer and frame type structural members in aircraft fuselages, and rib and spar type structural members in aircraft wings.
A substantial disadvantage of conventional type airframe construction is that wherever two structural members intersect one another, it is necessary to provide a cut-away portion in one of the structural members so as to accommodate the other structural member. Cutting away a portion of a structural member, of course, diminishes its load bearing capacity. Consequently, in conventional airframe construction it is necessary to provide additional reinforcing members to fasten the two structural members to one another at their point of intersection.
The principle of prior art airframe construction utilizing the aforementioned reinforcing members is schematically shown on FIGS. 1 and 2 of the drawings. On FIG. 1 a structural member incorporating a cut-away portion illustrates, e.g., a fore-and-aft positioned stringer of an aircraft fuselage, and another structural member illustrates a laterally extending frame member of the fuselage. The necessity of providing a cut-away portion in one of the structural members whenever two structural members "intersect" one another, or "attempt to occupy the same space", is not limited to airframe construction. A similar problem is encountered in the construction of boats, vehicle frames, buildings, etc. It is readily apparent to those skilled in the art that providing appropriately positioned cut-away portions and mounting the necessary reinforcing members or clips is time consuming and significantly contributes to the overall construction cost.
Although the state-of-the-art application of high strength, relatively light weight unidirectional composite materials has offered certain advantages, it has not, up to the present invention, resulted in an altogether different highly advantageous method or structure for joining two intersecting structural members to one another. With particular reference to FIG. 1, it is noted, that in the state-of-the-art airframe and the like composite structures, a cut-away portion is provided in one of the structural members as in conventional metal structures. Attachment of the additional reinforcing members or "clips" may be accomplished, however, by using a structural adhesive resin instead of welding, rivets, screws or bolts and nuts of a conventional metal construction.
Accordingly, there is a substantial need for the novel and unique high strength structure and method of the present invention which provides for a high strength junction of two or more substantially intersecting composite structural members.